JCM900 combo, volume down and up

[siggo]

Hi everybody,
I have a problem with a marshall jcm 100W combo.
A friend of mine gave me tha amp and said "try it", because he experienced a sudden decrease (and increase after some seconds) of output volume after a while he was playing (10/20 mins). I experienced the same problem after a while I was playing using the clean channel (spiltted to only 50W).
I think I'll take it simple first and I would like to replace the 2 pre-amp valves.
I have one JJ Tesla ECC83S and a Sovtek 12AX7 equivalent. I think they're OK.
What do you think?

P.S.: In what position V1 and V2 are looking to amp from back side?
I found shematic but not layout and I'm quite new to Marsall amp.

[generic_user]

does it have an effects loop, get a cable and patch the send and return jacks. check all your cables and maybe even check your guitar for good 'ol sake.

[siggo]

Yes, it has effects loop, I'll try with "tested" cable.
Same problem with different guitars (I don't think this is the problem).
Meanwhile I would like to replace two pre-amp tubes (very quick). Cuould you help me?

[StratTone]

Those 2 are fine replacements for test. V1 is the farthest one away from the power tubes.

[siggo]

Hi,
I replaced V1 with a JJ Tesla and I played for 1 hour without problem (but mainly on Overdrive channel).
I gave the amp to my friend and I'm waiting for him to play for a longer time.

Thanks everybody,

Ciao

[StratTone]

Hi,
I replaced V1 with a JJ Tesla and I played for 1 hour without problem (but mainly on Overdrive channel).
I gave the amp to my friend and I'm waiting for him to play for a longer time.

Thanks everybody,

Ciao

Good deal keep us posted.